Question

Consider this reaction at equilibrium: $$\operatorname{CO}(g)+\mathrm{Cl}_{2}(g) \rightleftharpoons \operatorname{COCl}_{2}(g)$$ Predict whether the reaction will shift left, shift right, or remain un- changed after each disturbance: \begin{equation}\begin{array}{l}{\text { a. } \mathrm{COCl}_{2} \text { is added to the reaction mixture. }} \\ {\text { b. } \mathrm{Cl}_{2} \text { is added to the reaction mixture. }} \\ {\text { c. } \mathrm{COCl}_{2} \text { is removed from the reaction mixture. }}\end{array}\end{equation}

Short answer

a. The reaction will shift left. b. The reaction will shift right. c. The reaction will shift right.

Step-by-step solution

- Analyze Disturbance a

When COCl2 is added to the reaction mixture, the concentration of the product increases. According to Le Chatelier's Principle, the system will attempt to counteract this change by shifting the equilibrium to decrease the concentration of added species. Therefore, the reaction will shift left, towards the reactants, to consume the added COCl2.

- Analyze Disturbance b

When Cl2 is added to the reaction mixture, the concentration of one of the reactants increases. Le Chatelier's Principle indicates that the system will react to this change by shifting the equilibrium position to reduce the concentration of the added reactant. Therefore, the reaction will shift right, towards the products, to consume the additional Cl2.

- Analyze Disturbance c

When COCl2 is removed from the reaction mixture, the concentration of the product decreases. According to Le Chatelier's Principle, the equilibrium will shift to replace the removed species. Therefore, the reaction will shift right, towards the products, to produce more COCl2.

Key concepts

Chemical Equilibrium

In a chemical reaction, when the rate of the forward reaction is equal to the rate of the reverse reaction, we reach a state called chemical equilibrium. At this point, the concentrations of reactants and products remain constant over time, although the reactions continue to occur on a microscopic level. It's important to understand that equilibrium does not mean the reactants and products are present in equal amounts; rather, it means their rates of formation are equally steady, leading to a balance. For example, in the equilibrium
\[ \text{CO}(g) + \text{Cl}_2(g) \rightleftharpoons \text{COCl}_2(g) \]
the formation rate of COCl2 is balanced by its decomposition rate back into CO and Cl2. It’s like a dance where the number of dancers entering the floor equals the number leaving it; the total number on the dance floor remains the same. Knowing this baseline is essential when studying how equilibria respond to changes, something described by Le Chatelier's Principle.

Reaction Shifts

When we consider reaction shifts, we're talking about the direction in which a chemical reaction moves when it's subjected to a change. Picture equilibrium as a perfectly balanced scale. If you add or remove weight from one side, the scale tips. Similarly, a balanced chemical reaction will shift to oppose any change made to the reactants or products.
When extra reactants are added, like in the case of Cl2 in our exercise, the reaction shifts towards the products to rebalance. This is known as a 'shift right'. Conversely, if products are added or reactants are taken away, the reaction will shift left, towards making more reactants. Understanding these shifts helps chemists control reactions to make them more efficient or to predict how a reaction mixture will change in different conditions.

Equilibrium Disturbance

The principle governing what happens when a chemical reaction at equilibrium is disturbed is called Le Chatelier's Principle. It states that when a system at equilibrium experiences a change in concentration, temperature, or pressure, the equilibrium will shift in a way that minimizes the disturbance. In reference to our exercise, adding COCl2 (a product) or removing COCl2 disturbs the equilibrium. When COCl2 is added, the equilibrium shifts left to consume the excess product, while removing COCl2 causes a shift right to produce more of it. It's like trying to quiet a seesaw: when too much weight is added to one side, you’d move some back to the other side to level it out. This adaptive quality of chemical reactions helps scientists and engineers design processes and products by meticulously controlling reaction conditions.